Molecular analysis of an aurea photosynthetic mutant (Su/Su) in tobacco: LHCP depletion leads to pleiotropic mutant phenotypes.

Su is a nuclear encoded, semi-dominant aurea mutation in Nicotiana tabacum L. The homozygous plants (Su/Su) are pale yellow and non-photosynthetic while the heterozygous (Su/+) are photosynthetically competent and have a yellow-green phenotype which is distinct from that of green wild-type plants (+/+). We have examined the RNA and protein levels for a number of nuclear and plastid encoded chloroplast proteins under high and low light plant growth conditions. Under high light conditions, the light-harvesting chlorophyll a/b binding proteins (LHCP) were undetectable in the homozygous Su/Su plants, and the large subunit (LSu) and the small subunit (SSu) of ribulose bisphosphate carboxylase (Rubisco) and cytochrome b559 were severely deficient. However, only the nuclear encoded cab and plastid encoded psbE mRNA (encoding LHCP and cytochrome b559 respectively) were reduced significantly. In heterozygous Su/+ plants, the level of LHCP was reduced to 25% of that in wild-type plants while cab and psbE mRNA, LSu, SSu and cytochrome b559 remained at normal levels, suggesting that LCHP is more immediately affected by the Su mutant gene product than the rest of the photosynthetic proteins and mRNA examined. Under low light conditions, the levels of cab and psbE mRNA, LSu, SSu and cytochrome b559 in homozygous Su/Su plants were equivalent to those in wild-type plants except LHCP which remained undetectable. Similarly, the LHCP level in low light grown Su/+ plants still remained at 25% of wild-type level. These results indicate that the decrease in LHCP is independent of light conditions and has not resulted from photooxidation, whereas the depletion of other proteins and mRNA examined under high light growth conditions is a consequence of photooxidative damage to Su/Su plastids. Furthermore, transgenic Su/Su and Su/+ plants with a cauliflower mosaic virus 35S (CaMV 35S)-cab construct constitutively maintained high levels of cab mRNA but displayed the same pattern of diminished LHCP accumulation as their non-transformed counterparts when grown under both high and low light conditions. These results indicate that the Su mutation primarily causes depletion of LHCP. The depletion of LHCP leads to photooxidative damage which results in decreased cab mRNA levels and other pleiotropic lesions in Su/Su plants.